OK, let's do this. We've got 615566 and 65603, and we're subtracting the second number from the first.

Step 1: We'll start by subtracting the digit 3 and the borrow 0 from 6 in column 1 and get 3.
3 is the first digit of our result.

Step 2: We'll start by subtracting the digit 0 and the borrow 0 from 6 in column 2 and get 6.
6 is the next digit of our result.

Step 3: We'll start by subtracting the digit 6 and the borrow 0 from 5 in column 3 and get -1.
We add -1 to 10 and get 9 as the next digit of the result.

Step 4: We'll start by subtracting the digit 5 and the borrow 1 from 5 in column 4 and get -1.
We add -1 to 10 and get 9 as the next digit of the result.

Step 5: We'll start by subtracting the digit 6 and the borrow 1 from 1 in column 5 and get -6.
We add -6 to 10 and get 4 as the next digit of the result.

Step 6: We'll start by subtracting the digit 0 and the borrow 1 from 6 in column 6 and get 5.
5 is the next digit of our result.

So, 615566 - 65603 = 549963.